Reproductive health is a window to overall health. Fifteen to 20% of couples have difficulty conceiving1; failures of the reproductive system thus affect a substantial population. Beyond fertility, sex steroids alter development and function of many systems, for example, bone, the central nervous system and the cardiovascular system. Episodic release of gonadotropin-releasing hormone (GnRH) is required for fertility in all vertebrates of both sexes, and frequency shifts are essential for female reproductive cycles. The goal of this proposal is to increase our fundamental understanding of the generation of episodic release of GnRH as a critical link to understanding and modulating fertility. The proposed work will define intrinsic properties of GnRH neurons, local network interactions, and reciprocal communication with postulated driver neurons that coexpress kisspeptin, neurokinin B and dynorphin and are located in the arcuate nucleus of the hypothalamus (KNDy neurons). Our working model is that intrinsic and network mechanisms interact to produce the final synchronized episodic output of the GnRH neurosecretory system. We will study this system using state-of- the-art electrophysiological approaches combined with electrochemical measures of GnRH release in transgenic mice in which GnRH neurons and/or KNDy neurons are identified by expression of green or red fluorescent proteins. We have three specific aims. In Aim 1, we will determine how intrinsic properties of GnRH neurons contribute to the firing of action potential bursts and the relation between activity and secretion in individual cells. We wil study how specific ionic conductances are altered as a burst of firing starts, proceeds and terminates, and how activity of a GnRH neuron relates to secretion from that cell. In Aim 2, we will examine how burst firing (time course of seconds) relates to GnRH secretion (time course of minutes) in networks. To define synchrony in the GnRH network, we will perform long-term recordings of firing activity of GnRH neuron pairs in combination with electrochemical measurement of GnRH release. This will establish how the activity-secretion relationship in the single cell (Aim 1) is altered when the neuron is in the network. We will study and model how interburst interval is altered to produce peaks in firing activity. We will identify these mechanisms via iterative hypothesis-generating modeling and experimental testing. Finally, we will probe interactions among intrinsic and network properties of GnRH neurons by examining input preferences of GnRH neurons in comparison with actual input patterns. In Aim 3, we will study KNDy neuron activity patterns, network interactions and reciprocal interactions with GnRH neurons. Recently, a working model has emerged that episodic release itself, the hallmark of the GnRH neurosecretory system, is itself driven from outside the GnRH network by KNDy neurons. We will directly test elements of the working hypothesis that KNDy neurons form a self-interactive network that drives GnRH activity through kisspeptin release. We also have preliminary data supporting the equally interesting hypothesis that GnRH feeds back to inhibit KNDy neuronal networks.